Conversion of carbon dioxide to methanol using bi-reforming of methane or natural gas

ABSTRACT

The invention provides for \ a method of forming methanol by combining a mixture of methane, water and carbon dioxide under reaction conditions sufficient to form a mixture of hydrogen and carbon monoxide. Hydrogen and carbon monoxide are reacted under conditions sufficient to form methanol. The molar ratio of hydrogen to carbon monoxide is at least two moles of hydrogen to one mole of carbon monoxide and the overall molar ratio between methane, water and carbon dioxide is about 3:2:1. Methane, carbon dioxide and water are bi-reformed over a catalyst. The catalyst includes a single metal, a metal oxide, a mixed catalyst of a metal and a metal oxide or a mixed catalyst of at least two metal oxides.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/850,501filed Sep. 5, 2007 which claims the benefit of application No.60/945,501 filed Jun. 21, 2007. Each document is expressly incorporatedherein by reference thereto.

BACKGROUND

Hydrocarbons are essential in modern life. Hydrocarbons are used as fueland raw material in various fields, including the chemical,petrochemical, plastics, and rubber industries. Fossil fuels, such ascoal, oil and natural gas, are composed of hydrocarbons with varyingratios of carbon to hydrogen. Despite their wide application and highdemand, fossil fuels also have limitations and disadvantages,particularly due to their finite reserve, irreversible combustion andcontribution to air pollution (and thus to global warming). Regardlessof these problems the more efficient use of still existing natural gassources is highly desirable. Further new sources and ways for recyclableand environmentally benign carbon fuels are needed.

One alternative frequently mentioned non-carbon fuel is hydrogen, andits use in the so-called “hydrogen economy.” Hydrogen is thought to bebeneficial as a clean fuel, producing only water when combusted. Freehydrogen, however, is not a natural primary energy source on earth, dueto its incompatibility with the atmospheric oxygen. It must be generatedfrom hydrocarbons or water, which is a highly energy-consuming process.Further, as hydrogen is produced from hydrocarbons or coal, any claimedbenefit of hydrogen as a clean fuel is outweighed by the fact that itsgeneration, mainly by reforming of natural gas, oil or coal to synthesisgas (“syn-gas” a mixture of CO and H₂), or the generation of electricityfor the electrolysis of water is far from clean, besides hydrogen isdifficult and costly to handle, transport and distribute. As it isextremely light, volatile and potentially explosive, it requireshigh-pressure equipment. The needed non-existent infrastructure alsonecessitates special materials to minimize diffusion and leakage, andextensive safety precautions to prevent explosions.

The continued importation of natural gas from far away and frequentlydifficult to access locations also necessitates its safe storage andtransportation particularly when involving liquefied natural gas (LNG).This necessitates transporting LNG at low temperatures in its liquidform exposing it to serious environmental and safety hazards. It issuggested that a more practical and safe alternative for LNG ismethanol, or dimethyl ether (DME), which are readily produced fromnatural gas. Methanol is the simplest liquid oxygenated hydrocarbon,differing from methane (CH₄) by a single additional oxygen atom.Methanol, also called methyl alcohol or wood alcohol, is a colorless,water-soluble liquid with a mild alcoholic odor. It is easy to store andtransport. It freezes at −97.6° C., boils at 64.6° C., and has a densityof 0.791 at 20° C.

Methanol is a convenient safe liquid easily obtained from existing coalor natural gas sources via methods developed and practiced since the1920's. However, these methods using conversion (reforming) of coal andsubsequently natural gas to syn-gas (a mixture of H₂ and CO) are highlyenergy consuming and produce large amount of CO₂ as a by-product. Thisis notably an economic disadvantage but also represents a seriousenvironmental problem by increasing one of the main greenhouse gascausing global warming.

Methanol not only represent a convenient and safe way to store andtransport energy, but together with its derived product dimethyl ether(DME), is an excellent fuel. Dimethyl ether is easily obtained frommethanol by dehydration or from methane (natural gas) with CO₂ via abi-reforming process. It is a particularly effective fuel for dieselengines because of its high cetane number and favorable combustionproperties. Methanol and dimethyl ether exceedingly blend well withgasoline or diesel oil to be used as fuels in internal combustionengines or electricity generators. One of the most efficient use ofmethanol is in fuel cells, particularly in direct methanol fuel cells(DMFC), in which methanol is directly oxidized with air to carbondioxide and water while producing electricity.

Contrary to gasoline, which is a complex mixture of many differenthydrocarbons and additives, methanol is a single simple chemicalcompound. It contains about half the energy density of gasoline, meaningthat two liters of methanol provide the same energy as a liter ofgasoline. Even though the energy content of methanol is lower, it has ahigher octane rating of 100 (average of the research octane number (RON)of 107 and motor octane number (MON) of 92), which means that thefuel/air mixture can be compressed to a smaller volume before beingignited. This allows the engine to run at a higher compression ratio of10-11 to 1 more efficiently than the 8-9 to 1 ratio of agasoline-powered engine. Efficiency is also increased by methanol's(oxygenates') higher “flame speed,” which enables faster, more completefuel combustion in the engines. These factors explain the highefficiency of methanol despite its lower energy density than gasoline.Further, to render methanol more ignitable even under the most frigidconditions, methanol is mixed with gasoline, and other volatilecomponents or with a device to vaporize or atomize methanol. Forexample, an effective automotive fuel comprised by adding methanol togasoline with the fuel having a minimum gasoline content of at least 15%by volume (M85 fuel) so that the engine can readily start even in lowtemperature environments were commercially used in the US in the 1980's.M20fuel (with 20 volume % methanol) is also being introduced. Similarly,dimethyl ether (DME) mixed with diesel oil or in household use as asubstitute of natural gas or LPG is of commercial interest. Thesemixtures are not only efficient fuels but conserve or replace decreasingoil resources. The amount of methanol or dimethyl ether added can bedetermined depending upon the specific condition and needs.

Methanol has a latent heat of vaporization of about 3.7 times higherthan gasoline, and can absorb a significantly larger amount of heat whenpassing from liquid to gaseous state. This helps to remove heat awayfrom the engine and enables the use of an air-cooled radiator instead ofa heavier water-cooled system. Thus, compared to a gasoline-powered car,a methanol-powered engine provides a smaller, lighter engine block,reduced cooling requirements, and better acceleration and mileagecapabilities. Methanol and DME are also more environmentally-friendlythan gasoline or diesel oil, and produce low overall emissions of airpollutants such as certain hydrocarbons, NO_(x), SO₂ and particulates.

Methanol is also one of the safest fuels available. Compared togasoline, its physical and chemical properties significantly reduce therisk of fire. Methanol has lower volatility, and methanol vapor must befour times more concentrated than gasoline for ignition to occur. Evenwhen ignited, methanol burns about four times slower than gasoline,releases heat only at one-eighth the rate of gasoline fire, and is farless likely to spread to surrounding ignitable materials because of thelow radiant heat output. It has been estimated by the EPA that switchingfrom gasoline to methanol would reduce incidence of fuel-related fire by90%. Methanol burns with a colorless flame, but additives can solve thisproblem. As methanol is completely miscible with water not only it isenvironmentally readily decomposed in nature but in contrast to ethanolthere are no strict requirements needed to keep it dry to avoid phaseseparation from gasoline.

Methanol also provides an attractive and more environmentally-friendlyalternative to diesel fuel. It does not produce smoke, soot, orparticulates when combusted, in contrast to diesel fuel, which generallyproduces polluting particles during combustion. It also produce very lowemissions of NO_(x) because it burns at a lower temperature than diesel.Furthermore, it has a significantly higher vapor pressure compared todiesel fuel, and the higher volatility allows easy start even in coldweather, without producing smoke typical of cold start with aconventional diesel engine. If desired, additives or ignition improvers,such as octyl nitrate, tetrahydrofurfuryl nitrate, peroxides or higheralkyl ethers, can be added to bring methanol's cetane rating to thelevel closer to diesel. Methanol is also used in the manufacture ofbiodiesel fuels by esterification of fatty acids.

As mentioned, the closely related derivative of methanol, which is ahighly desirable alternative fuel, is dimethyl ether. Dimethyl ether(CH₃OCH₃), the simplest of all ethers, is a colorless, nontoxic,non-corrosive, non-carcinogenic and environmentally friendly chemicalthat is mainly used today as an aerosol propellant in spray cans, inplace of the banned CFC gases. Dimethyl ether has a boiling point of−25° C., and is a gas under ambient conditions. Dimethyl ether is,however, easily handled as a liquid and stored in pressurized tanks,much like liquefied petroleum gas (LPG). The interest in dimethyl etheras alternative fuel lies in its high cetane rating of 55 to 60, which ismuch higher than that of methanol and is also higher than the cetanerating of 40 to 55 of conventional diesel fuels. The cetane ratingindicates that dimethyl ether is effectively used in diesel engines.Advantageously, dimethyl ether, like methanol, is clean burning, andproduces no soot particulates, black smoke or SO₂, and only very lowamounts of NO_(x) and other emissions even without after-treatment ofits exhaust gas. Some of the physical and chemical properties of DME, incomparison to diesel fuel, are shown in Table 1.

TABLE 1 Comparison of the physical properties of DME and diesel fuelDimethyl Ether Diesel Fuel Boiling point ° C. −24.9 180-360 Vaporpressure at 20° C. (bar) 5.1 — Liquid density at 20° C. (kg/m) 668840-890 Heating value (kcal/kg) 6,880 10,150 Cetane number 55-60 40-55Autoignition temperature (° C.) 235 200-300 Flammability limits in air(vol %)  3-17 0.6-6.5

Currently, dimethyl ether is produced by the direct dehydration ofmethanol according to the following reaction:2CH₃OH→CH₃OCH₃→H₂O

Another methanol derivative is dimethyl carbonate (DMC), which can beobtained by converting methanol with phosgene or by oxidativecarbonylation of methanol. DMC has a high cetane rating, and can beblended into diesel fuel in a concentration up to 10%, reducing fuelviscosity and improving emissions.

Methanol and its derivatives, e.g., dimethyl ether, DMC, and biodieselfuel (esters of naturally occurring unsaturated acids) already havesignificant and expanding uses. They can be used, for example, as asubstitute for gasoline and diesel fuel in ICE-powered cars with onlyminor modifications to the existing engines and fuel systems. Methanolcan also be used in fuel cells, for fuel cell vehicles (FCVs), which areconsidered to be the best alternatives to ICEs in the transportationfield. DME is also starting to be used in admixture to LNG and LPG indomestic and industrial fuel uses.

Methanol can also be used in reforming to produce hydrogen. In an effortto address the problems associated with hydrogen storage anddistribution, suggestions have been made to use liquids rich in hydrogensuch as gasoline or methanol as a source of hydrogen in vehicles via anon-board reformer. It was emphasized that methanol is the safest of allmaterials available for such hydrogen production. Further, because ofthe high hydrogen content of liquid methanol, even compared to purecryogenic hydrogen (98.8 g of hydrogen in a liter of methanol at roomtemperature compared to 70.8 g in liquid hydrogen at about −253° C.),methanol is an excellent carrier of hydrogen fuel. The absence of C—Cbonds in methanol, which are more difficult to break, facilitates itstransformation to pure hydrogen in 80 to 90% efficiency.

In contrast to a pure hydrogen-based storage system, a reformer systemis compact, containing on a volume basis more hydrogen than even liquidhydrogen, and is easy to store and handle without pressurization. Amethanol steam reformer is also advantageous in allowing operation at amuch lower temperature (250° C. to 350° C.) and for being better adaptedto on-board applications. Furthermore, methanol contains no sulfur, acontaminant for fuel cells, and no nitrogen oxides are formed from amethanol reformer because of the lower operating temperature.Particulate matter and NO_(x) emissions are virtually eliminated, andother emissions are minimal. Moreover, methanol allows refueling to beas quick and easy as with gasoline or diesel fuel. Thus, an on-boardmethanol reformer enables rapid and efficient delivery of hydrogen fromliquid fuel that can be easily distributed and stored in the vehicle. Todate, methanol is the only liquid fuel that has been demonstrated on apractical scale to be a suitable liquid fuel for a reformer to producehydrogen for use in a fuel cells for transportation applications.

In addition to on-board reforming, methanol also enables convenientproduction of hydrogen in fueling stations for refueling hydrogen fuelcell vehicles. A fuel cell, an electrochemical device that converts freechemical energy of fuel directly into electrical energy, provides ahighly efficient way of producing electricity via catalyticelectrochemical oxidation. For example, hydrogen and oxygen (air) arecombined in an electrochemical cell-like device to produce water andelectricity. The process is clean, with water being the only byproduct.However, because hydrogen itself must first be produced in anenergy-consuming process, by electrolysis or from a hydrocarbon source(fossil fuel) with a reformer, hydrogen fuel cells are still necessarilylimited in their utility.

A system for producing high purity hydrogen has been developed by steamreforming of methanol with a highly active catalyst, which allowsoperation at a relatively low temperature (240° C. to 290° C.) andenables flexibility in operation as well as rapid start-up and stop.These methanol-to-hydrogen (MTH) units, ranging in production capacityfrom 50 to 4000 m³H₂ per hour, are already used in various industries,including the electronic, glass, ceramic, and food processingindustries, and provide excellent reliability, prolonged life span, andminimal maintenance. As described above, operating at a relatively lowtemperature, the MTH process has a clear advantage over reforming ofnatural gas and other hydrocarbons which must be conducted at above 600°C., because less energy is needed to heat methanol to the appropriatereaction temperature.

The usefulness of methanol has led to the development of other reformingprocesses, for example, a process known as oxidative steam reforming,which combines steam reforming, partial oxidation of methanol, usingnovel catalyst systems. Oxidative steam reforming produces high purityhydrogen with zero or trace amounts of CO, at high methanol conversionand temperatures as low as 230° C. It has the advantage of being,contrary to steam reforming, an exothermic reaction, therefore,minimizing energy consumption. There is also auto thermal reforming ofmethanol, which combines steam reforming and partial oxidation ofmethanol in a specific ratio and addresses any drawback of an exothermicreaction by producing only enough energy to sustain itself. Auto thermalreforming is neither exothermic nor endothermic, and does not requireany external heating once the reaction temperature is reached. Despitethe aforementioned possibilities, hydrogen fuel cells must use highlyvolatile and flammable hydrogen or reformer systems.

U.S. Pat. No. 5,599,638 discloses a simple direct methanol fuel cell(DMFC) to address the disadvantages of hydrogen fuel cells. In contrastto a hydrogen fuel cell, the DMFC is not dependent on generation ofhydrogen by processes such as electrolysis of water or reformation ofnatural gas or hydrocarbons. The DMFC is also more cost effectivebecause methanol, as a liquid fuel, does not require cooling at ambienttemperatures or costly high pressure infrastructure and can be used withexisting storage and dispensing units, unlike hydrogen fuel, whosestorage and distribution requires new infrastructure. Further, methanolhas a relatively high theoretical volumetric energy density compared toother systems such as conventional batteries and the H₂-PEM (PEM: protonexchange membrane) fuel cell. This is of great importance for smallportable applications (cellular phones, laptop computers, etc.), forwhich small size and weight of energy unit is desired.

DMFC offers numerous benefits in various areas, including thetransportation sector. By eliminating the need for a methanol steamreformer, DMFC significantly reduces the cost, complexity and weight ofthe vehicle, and improves fuel economy. A DMFC system is also comparablein its simplicity to a direct hydrogen fuel cell, without the cumbersomeproblems of on-board hydrogen storage or hydrogen producing reformers.Because only water and CO₂ are emitted, emissions of other pollutants(e.g., NO_(x), particulate matter, SO₂, etc.) are eliminated. Directmethanol fuel cell vehicles are expected to be of low emission (ZEV),and use of methanol fuel cell vehicles will greatly eliminate airpollutants from vehicles in the long term. Further, unlike internalcombustion engine vehicles, the emission profile is expected to remainnearly unchanged over time. New fuel cell membranes based on hydrocarbonor hydrofluorocarbon materials with reduced cost and crossovercharacteristics have been developed that allow room temperatureefficiency of about 34%.

Methanol and dimethyl ether provide a number of important advantages astransportation fuels. By contrast to hydrogen, methanol storage does notrequire any energy intensive procedures for pressurization orliquefaction. Because it is a liquid at room temperature, it can beeasily handled, stored, distributed and carried in vehicles. It can actas an ideal hydrogen carrier for fuel cell vehicles through on-boardmethanol reformers or can be used directly in DMFC vehicles. Dimethylether although gaseous at room temperature can be easily stored undermodest pressure and used effectively in admixture with diesel fuels andliquefied natural gas (LNG), or used in residential gas mixtures.

Methanol is also an attractive liquid fuel for static applications. Forexample, methanol can be used directly as fuel in gas turbines togenerate electric power. Gas turbines typically use natural gas or lightpetroleum distillate fractions as fuel. Compared to such fuels, methanolcan achieve higher power output and lower NO_(x) emissions because ofits lower flame temperature. Since methanol does not contain sulfur, SO₂emissions are also eliminated. Operation on methanol offers the sameflexibility as on natural gas and distillate fuels, and can be performedwith existing turbines, originally designed for natural gas or otherfossil fuels, after relatively easy modification. Methanol is also anattractive fuel since fuel-grade methanol, with lower production costthan higher purity chemical-grade methanol, can be used in turbines.Because the size and weight of a fuel cell is of less importance instatic applications than mobile applications, various fuel cells otherthan PEM fuel cells and DMFC, such as phosphoric acid, molten carbonateand solid oxide fuel cells (PAFC, MCFC, and SOFC, respectively), canalso be used.

In addition to use as fuels, methanol, dimethyl ether and derivedchemicals have significant applications in the chemical industry. Today,methanol is one of the most important feedstock in the chemicalindustry. The majority of the some 35 million tons of the annuallyproduced methanol are used to manufacture a large variety of chemicalproducts and materials, including basic chemicals such as formaldehyde,acetic acid, MTBE (although it is increasingly phased out forenvironmental reasons), as well as various polymers, paints, adhesives,construction materials, and others. Worldwide, methanol is used toproduce formaldehyde (38%), methyl-tent-butyl ether (MTBE, 20%) andacetic acid (11%). Methanol is also a feedstock for chloromethanes,methylamines, methyl methacrylate, and dimethyl terephthalate, amongothers. These chemical intermediates are then processed to manufactureproducts such as paints, resins, adhesives, antifreeze, and plastics.Formaldehyde, produced in large quantities from methanol, is mainly usedto prepare phenol-, urea- and melamine-formaldehyde and polyacetalresins as well as butanediol and methylene bis(4-phenyl isocyanate) MDIfoam, which is used as insulation in refrigerators, doors, and in cardashboards and bumpers. Formaldehyde resins are predominantly used asadhesives in a wide variety of applications, e.g., manufacture ofparticle boards, plywood and other wood panels. Examples of majormethanol-derived chemical products and materials produced are listed inFIG. 1.

In producing basic chemicals, raw material feedstock constitutestypically up to 60-70% of the manufacturing costs. The cost of feedstocktherefore plays a significant economic role and its continuedavailability is essential. Because of its economic and long rangeavailability advantages, methanol is considered a potential primefeedstock for processes currently utilizing more expensive feedstocksuch as ethylene and propylene, to produce chemicals including aceticacid, acetaldehyde, ethanol, ethylene glycol, styrene, and ethylbenzene,and various synthetic hydrocarbon products. For example, directconversion of methanol to ethanol can be achieved using a rhodium-basedcatalyst, which has been found to promote the reductive carbonylation ofmethanol to acetaldehyde with selectivity close to 90%, and a rutheniumcatalyst, which further reduces acetaldehyde to ethanol. Anotherfeasible way to produce ethanol from methanol involves conversion ofethylene follow by hydration according to the overall reaction:2CH₃OH→C₂H₅OH+H₂O

Producing ethylene glycol via methanol oxidative coupling instead ofusing ethylene as feedstock is also pursued, and significant advancesfor synthesizing ethylene glycol from dimethyl ether, obtained bymethanol dehydration, have also been made.

Conversion of methanol to olefins such as ethylene and propylene, alsoknown as methanol to olefin (MTO) technology, is particularly promisingconsidering the high demand for olefins, especially in polyolefin andsynthetic hydrocarbon products production. The MTO technology ispresently a two-step process, in which natural gas is converted tomethanol via syn-gas and methanol is then transformed to olefin. It isconsidered that in the process, methanol is first dehydrated to dimethylether (DME), which then reacts to form ethylene and/or propylene. Smallamounts of butenes, higher olefins, alkanes, and aromatics are alsoformed.

Various catalysts, include without limitation, synthetic aluminosilicatezeolite catalysts, such as ZSM-5 (a zeolite developed by Mobil),silicoaluminophosphate (SAPO) molecular sieves such as SAPO34 andSAPO-17 (UOP), as well as bi-functional supported acid-base catalystssuch as tungsten oxide over alumina WO₃/Al₂O₃, have been found to beactive in converting methanol to ethylene and propylene at a temperaturebetween 250° C. and 400° C. The nature and amount of the end productdepend on the type of the catalyst, contact time and other factors ofthe process used. Depending on the operating conditions, the weightratio of propylene to ethylene can be modified between about 0.77 and1.33, allowing considerable flexibility. For example, when using SAPO-34catalyst according to an MTO process developed by UOP and Norsk Hydro,methanol is converted to ethylene and propylene at more than 80%selectivity, and also to butene, a valuable starting material for anumber of products, at about 10%. When using an MTO process developed byLurgi with ZSM-5 catalysts, mostly propylene is produced at yields above70%. A process developed by ExxonMobil, with ZSM-5 catalyst, produceshydrocarbons in the gasoline and/or distillate range at selectivitygreater than 95%.

There is also a methanol to gasoline (MTG) process, in which medium-porezeolites with considerable acidity, e.g., ZSM-5, are used as catalysts.In this process, methanol is first dehydrated to an equilibrium mixtureof dimethyl ether, methanol and water over a catalyst, and this mixtureis then converted to light olefins, primarily ethylene and propylene.The light olefins can undergo further transformations to higher olefins,C₃-C₆ alkanes, and C₆-C₁₀ aromatics such as toluene, xylenes, andtrimethylbenzene.

With decreasing oil and natural gas reserves, it is inevitable thatsynthetic hydrocarbons would play a major role. Thus, methanol-basedsynthetic hydrocarbons and chemicals available through MTG and MTOprocesses are assuming increasing importance in replacing oil andgas-based materials. The listed uses of methanol in FIG. 1 are onlyillustrative and not limiting.

Methanol can also be used as a source of single cell proteins. A singlecell protein (SCP) refers to a protein produced by a microorganism whichdegrades hydrocarbon substrates while gaining energy. The proteincontent depends on the type of microorganism, e.g., bacteria, yeast,mold, etc. The SCP has many uses, including uses as food and animalfeed.

Considering the numerous uses of methanol and dimethyl ether, it isclearly desirable to have improved and efficient methods for theirproduction. Currently, methanol is almost exclusively made fromsynthesis gas obtained from incomplete combustion (or catalyticreforming) of fossil fuel, mainly natural gas (methane) and coal.

Methanol can also be made from renewable biomass, but such methanolproduction also involves syn-gas and may not be energetically favorableand limited in terms of scale. As used herein, the term “biomass”includes any type of plant or animal material, i.e., materials producedby a life form, including wood and wood waste, agricultural crops andtheir waste byproducts, municipal solid waste, animal waste, aquaticplants, and algae. The method of transforming biomass to methanol issimilar to the method of producing methanol from coal, and requiresgasification of biomass to syn-gas, followed by methanol synthesis bythe same processes used with fossil fuel. Use of biomass also presentsother disadvantages, such as low energy density and high cost ofcollecting and transporting bulky biomass. Although recent improvementsinvolving the use of “biocrude,” black liquid obtained from fastpyrolysis of biomass, is somewhat promising, more development is neededfor commercial application of biocrude.

The presently existing methods of producing methanol involve syn-gas.Syn-gas is a mixture of hydrogen, carbon monoxide and carbon dioxide,and produces methanol over a heterogeneous catalyst according to thefollowing reactions:CO+2H₂→CH₃OH ΔH_(298K)=−21.7 kcal/molCO₂+3H₂→CH₃OH+H₂O ΔH_(298K)=−9.8 kcal/molCO₂+H₂→CO+H₂O ΔH_(298K)=11.9 kcal/mol

The first two reactions are exothermic with heat of reaction equal toabout 21.7 kcal·mol/l and about 9.8 kcal·mol/l, respectively, and resultin a decrease in volume. Conversion to methanol is favored by increasingthe pressure and decreasing the temperature according to Le Chatelier'sprinciple. The third equation describes the endothermic reverse watergas shift reaction (RWGSR). Carbon monoxide produced in the thirdreaction can further react with hydrogen to produce methanol. The secondreaction is simply the sum of the first and the third reactions. Each ofthese reactions is reversible, and is therefore limited by thermodynamicequilibrium under the reaction conditions, e.g., temperature, pressureand composition of the syn-gas.

Synthesis gas for methanol production can be obtained by reforming orpartial oxidation of any carbonaceous material, such as coal, coke,natural gas, petroleum, heavy oil, and asphalt. The composition ofsyn-gas is generally characterized by the stoichiometric number S,corresponding to the reaction shown below.

Ideally, S should be equal to or slightly above 2. A value above 2indicates excess hydrogen, while a value below 2 indicates relativehydrogen deficiency. Reforming of feedstock having a higher H/C ratio,such as propane, butane or naphthas, leads to S values in the vicinityof 2, ideal for the conversion to methanol. When coal is used, however,additional treatment is required to obtain an optimal S value. Synthesisgas from coal requires treatment to avoid formation of undesiredbyproducts.

The most widely used technology to produce syn-gas for methanolsynthesis is steam reforming. In this process, natural gas (of whichmethane is the major component) is reacted in a highly endothermicreaction with steam over a catalyst, typically based on nickel, at atemperature of about 800° C. to about 1,000° C., and a pressure of about20 atm to 30 atm) to form CO and H₂. A part of the CO formed reactsconsequently with steam in the water gas shift reaction (WGS) to yieldmore H₂ and also CO₂. The gas obtained is thus a mixture of H₂, CO andCO₂ in various concentrations depending on the reaction conditions, suchas temperature, pressure and H₂O/CH₄ ratio according to the followingreactions:CH₄+H₂O→CO+3H₂ ΔH_(298K)=49.1 kcal/molCO+H₂O→CO₂+H₂ ΔH_(298K)=−9.8 kcal/mol

Since the overall methane steam reforming process is highly endothermic,heat must be supplied to the system by burning a part of the natural gasused as the feedstock. The stoichiometric number S obtained by steamreforming of methane is close to 3, much higher than the desired valueof 2. This can generally be corrected by addition of CO₂ to the steamreformer's exit gas or use of excess hydrogen in some other process suchas ammonia synthesis. However, natural gas is still the preferredfeedstock for methanol production because it offers high hydrogencontent and, additionally, the lowest energy consumption, capitalinvestment and operating costs. Natural gas also contains fewerimpurities such as sulfur, halogenated compounds, and metals which maypoison the catalysts used in the process.

The existing processes invariably employ extremely active and selectivecopper-based catalysts, differing only in the reactor design andcatalyst arrangement. Because only part of the syn-gas is converted tomethanol after passing over the catalyst, the remaining syn-gas isrecycled after separation of methanol and water. There is also a morerecently developed liquid phase process for methanol production, duringwhich the syn-gas is bubbled into the reaction mixture. Although theexisting processes have methanol selectivity greater than 99% and energyefficiency above 70%, crude methanol leaving the reactor still containswater and other impurities, such as dissolved gases (e.g., methane, CO,and CO₂), dimethyl ether, methyl formate, acetone, higher alcohols(ethanol, propanol, butanol), and long-chain hydrocarbons. Commercially,methanol is available in three grades of purity: fuel grade, “A” grade,generally used as a solvent, and “AA” or chemical grade. Chemical gradehas the highest purity with a methanol content exceeding 99.85% and isthe standard generally observed in the industry for methanol production.The syn-gas generation and purification steps are critical to theexisting processes, and the end result would largely depend on thenature and purity of the feedstock. To achieve the desired level ofpurity, methanol produced by the existing processes is usually purifiedby sufficient distillation. Another major disadvantage of the existingprocess for producing methanol through syn-gas is the energy requirementof the first highly endothermic steam reforming step. The process isalso inefficient because it involves transformation of methane in anoxidative reaction to CO (and some CO₂), which in turn must be reducedto methanol.

Another way to produce syn-gas from methane is through the partialoxidation reaction with insufficient oxygen, which can be performed withor without a catalyst. This reaction is exothermic and is conducted athigh temperature of about 1,200° C. to about 1,500° C. The problem withpartial oxidation is that the products, CO and H₂ are readily furtheroxidized to form undesired CO₂ and water in highly exothermic reactionsleading to S values typically well below 2 and contributing to CO₂induced global warming. The following reactions are illustrative of theprocess.CH₄+½O₂→CO+2H₂ ΔH_(298K)=−8.6 kcal/molCO+½O₂→CO₂ ΔH_(298K)=−67.6 kcal/mol

To produce syn-gas without either consuming or producing much heat,modern plants are usually combining exothermic partial oxidation withendothermic steam reforming in order to have an overallthermodynamically neutral reaction while obtaining a syn-gas with acomposition suited for methanol synthesis (S close to 2). In thisprocess, called autothermal reforming, heat-produced by the exothermicpartial oxidation is consumed by the endothermic steam reformingreaction. Partial oxidation and steam reforming can be conductedseparately or simultaneously in the same reactor by reacting methanewith a mixture of steam and oxygen. The process as mentioned however,produces large amounts of CO₂ necessitating its costly sequestering orventing into the atmosphere. Any carbon containing fuel or derivedsynthetic hydrocarbon product when oxidatively used results in theformation of carbon dioxide and thus is not renewable on the human timescale. There is an essential need to make carbon fuels renewable andthus also environmentally neutral to minimize their harmful effect onglobal warming.

The selective conversion and recycling of carbon dioxide to methanolwithout generating unwanted by-products is thus a major challenge and amuch desired practical goal. There is a great need to effectively andeconomically produce methanol from carbon dioxide with high selectivityand yield of conversion.

SUMMARY OF THE INVENTION

The invention now provides novel methods for converting methane andcarbon dioxide to methanol without any release of carbon dioxide to theatmosphere or without by-product formation or the use of hydrogen toform water.

In one embodiment, the invention provides for a method of formingmethanol, by combining a methane, water and carbon dioxide, preferablyin a mixture, in single or multiple steps under reaction conditionssufficient to form a molar mixture of hydrogen and carbon monoxide in aspecific ratio of at least two moles of hydrogen to one mole of carbonmonoxide, and reacting the mixture of hydrogen and carbon monoxide underconditions sufficient to exclusively form methanol. The molar ratio ofhydrogen to carbon monoxide is preferably between 2:1 and 2.1:1. Themolar ratio between the methane, water and carbon dioxide is about3:2:1.

In another embodiment, the invention provides for a method of formingmethanol by reacting methane and water under steam reforming reactionconditions sufficient to form hydrogen and carbon monoxide, reactingmethane and carbon dioxide under dry reforming reaction conditionssufficient to form hydrogen and carbon monoxide, forming a mixture ofthe combined hydrogen and carbon monoxide in a molar ratio of at least 2moles of hydrogen to one mole of carbon monoxide, and reacting thecarbon monoxide and hydrogen under reaction conditions sufficient toexclusively form methanol. The molar ratio of hydrogen to carbonmonoxide is preferably is between 2:1 and 2.1:1. The overall combinedmolar ratio involving two separate steps between methane, water andcarbon dioxide is also about 3:2:1.

Methanol is formed over a catalyst on a support at a temperature of fromabout 800° C. to 1100° C. A preferred catalyst includes a single metalcatalyst, a single metal oxide catalyst, a mixed catalyst of a metal anda metal oxide or a mixed catalyst of at least two metal oxides. Thecatalyst includes V, Ti, Ga, Mg, Cu, Ni, Mo, Bi, Fe, Mn, Co, Nb, Zr, Laor Sn or an oxide thereof. The catalyst may be present on a support of ahigh surface or nanostructured oxide, such as fumed alumina or fumedsilica. In a specific embodiment, the catalyst is NiO or a mixedcatalyst of NiO, V₂O₅:Ni₂O₃, Ni₂V₂O₇ and Ni₃V₂O₅. In a more specificembodiment, the catalyst is NiO supported on fumed alumina or NiO/V₂O₅supported on a fumed silica surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and benefits of the invention will become more evident fromreview of the following detailed description of illustrative embodimentsand the accompanying drawings, wherein:

FIG. 1 shows known examples of methanol-derived chemical products andmaterials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to processes for the conversion of carbon dioxidefrom any carbon dioxide source, methane from any methane source such asnatural gas, coal bed methane, methane hydrate or any other sources tomethanol or dimethyl ether. These processes of conversion are referredto as a bi-reforming processes and utilize a specific combination ofsteam (H₂O) and dry (CO₂) reforming of methane, practiced in two stepsor combined into a single step. The method comprises reacting methane ornatural gas under a combination of conditions of steam (wet) and dry(CO₂) reforming in a specific molar ratio of reactants sufficient toform a mixture of hydrogen/carbon dioxide (H₂/CO) in a molar ratio ofabout 2:1, preferably between 2:1 and 2.1:1, and most preferably about2.05:1; the ratios that are sufficient to convert such mixture of H₂ andCO exclusively to methanol or dimethyl ether. Advantageously, thereactants or mixture of reactants is treated without separation of itscomponents to convert substantially all the reactants to methyl alcoholor, if desired, to dimethyl ether without the production of anyby-products. Any unreacted starting or intermediate products can bereadily recovered and recycled.

Methanol and dimethyl ether formed by the processes described herein canfind utility in numerous applications, either alone, or upon subsequentconversion to other products. Without being limiting, methanol, dimethylether and their derived products can be used as synthetic ICE fuels,effective diesel fuels (including mixing varied amounts of DME dimethylether with conventional diesel fuel), gasoline-methanol mixed fuels(prepared by adding methanol to gasoline with the fuel having a minimumgasoline content of at least 15% by volume). Without being limited as toother uses, methanol and/or dimethyl ether are convenient energy storageand transportation materials in order to minimize or eliminate thedisadvantages or dangers inherent in the use and transportation of LNGor LPG. Dimethyl ether is also a convenient household gas to replacenatural gas. They are also convenient raw materials for producingolefins (ethylene, propylene etc.) synthetic hydrocarbons, theirproducts and materials, even for preparing single cell proteins forhuman or animal consumption.

The steps of the process of the invention for the formation of methanolare illustrated by the following reactions:

The bi-reforming process of producing methanol can be practiced bycarrying out steps A and B separately. The products of reforming ofsteps A and B are mixed together before being introduced into themethanol producing step D. The steam reforming step is carried byreacting methane and water in an equal molar ratio over a catalystbetween 800° C. and 1000° C. The dry reforming step is carried byreacting methane and carbon dioxide in an equal molar ratio over acatalyst between 800° C. and 850° C.

The bi-reforming process of producing methanol can also be practiced bycombining the two reforming steps A and B into a single reforming stepby reacting methane, water and carbon dioxide in a molar ratio of about3:2:1 over a catalyst between 800° C. and 1100° C. In many places,natural gas sources also contain substantial amount of CO₂.

In one embodiment of the invention, a specific combination of steam anddry reforming of methane is used to achieve a molar ratio of H₂ and COof at least 2 moles hydrogen to 1 mole of carbon monoxide for theconversion to methanol. In another embodiment, methane is treated withwater and carbon dioxide in a molar ratio of about 3:2:1 with atemperature range from about 800° C. to about 1100° C., preferably fromabout 800° C. to about 850° C. To allow conversion, a catalyst orcombination of catalysts can be used. These include any suitable metalor metal oxide, including without limitation a metal such as V, Ti, Ga,Mg, Cu, Ni, Mo, Bi, Fe, Mn, Co, Nb, Zr, La or Sn, and correspondingoxides of such metals. The catalysts may be used as a single metal, or acombination of a metal and metal oxide, or a combination of metaloxides, supported on a suitable support such as a high surface areananostructured oxide support such as fumed silica or fumed alumina. Byway of example, NiO, metal-metal oxides such as Ni—V₂O₅, (M₂O₃—V₂O₅),and NiO:V₂O₅, as well as mixed oxides such as Ni₂V₂O₇ and Ni₃V₂O₈ can beused. One of skill in the art would immediately appreciate that a numberof other related metal and metal oxide catalysts, and theircombinations, can also be used. Suitable reactors for the conversionreactions can also be used. For example, a continuous flow reactor underthe appropriate reaction conditions can be used for the reactions toproceed to completion either at ambient pressure or high pressure.

Carbon dioxide is not sequestered or released into the atmosphere andmethane is completely converted to methanol without producing anyby-product. This provides for significant economical and environmentaladvantages. By contrast with the processes described herein, thetri-reforming process of methane in which a synergetic combination ofdry reforming, steam reforming and partial oxidation of methane iscarried out in a single step, but produces by-products (CO₂ and H₂O) inthe oxidation step. By contrast with the tri-reforming process, theprocess of the invention provides for, control, high selectivity andyield of the conversion of carbon dioxide to methanol without anyby-products and without encountering the difficulties and having thedisadvantages associated with concurrent partial oxidation resulting inundesirable excess carbon dioxide and water.

The bi-reforming processes of the invention can be used for thepreparation of dimethyl ether without water formation as a by-product,as is the case in the presently used dehydration of methanol. Thisprovides an additional advantage as compared to the dry reformingprocess of producing methane as it gives only a 1:1 molar mixture of COand H₂ and is not suitable without modifications to the production ofdimethyl ether as illustrated by the following reaction.CH₄+CO₂→2CO+2H₂

For the production of dimethyl ether, water obtained from thedehydration of methanol can be recycled and reacted with carbon dioxideand methane with no by-product (H₂O or CO₂) formation in the overallprocess. Water removal is achieved over a suitable dry silica catalystor a polymeric perfluoroalkanesulfonic acid catalyst at a temperature offrom about 100° C. to 200° C. An example of such catalyst is Nafion-H.

The steps of the process of the invention for the production of dimethylether are illustrated by the following reactions:3CH₄+CO₂→2CH₃OCH₃3CH₄+2H₂O+CO₂→4CO+8H₂4CO+8H₂+4CH₃OH→2CH₃OCH₃+2H₂O

In an embodiment of the invention, the water formed during thedehydration of methanol is reacted with CH₄ and CO₂ of about 2:3:1overall molar ratio to form dimethyl ether. With water recycling,dimethyl ether is formed using methane and CO₂ in an overall ratio ofabout 3:1.

The dry-reforming process of the invention can also be directly appliedto natural gas (mixture of hydrocarbons) itself to form methanol ordimethyl ether in a separate step or in a single step with properselection of mixing to obtain the needed H₂ and CO molar mixture of atleast 2 moles of hydrogen to one mole of carbon monoxide required forthe production of methanol. Application to natural gas is illustrated bythe following reaction:3C_(n)H_((2n+2))+(3n−1)H₂O+CO₂→(3n+1)CO+(6n+2)H₂→4nCH₃OH

The processes of the invention have significant advantages over the useof syn-gas as it would apply to the production of methanol. Syn-gas ofvarying compositions can be produced by a variety of reactions. It isgenerally produced by the reaction of coal, methane, natural gas withsteam (steam reforming). As mentioned in Step B, syn-gas can also beproduced by the reaction of CO₂ with methane or natural gas in a processcalled “CO₂” or “dry” reforming, because it does not involve steam. Thegas mixture produced from methane and CO₂, has an H₂/CO ratio closeto 1. Therefore, for methanol production, hydrogen generated from othersources must be added to obtain the molar ratio of about 2:1. There isno upper limit for this ratio as long as there is an excess of hydrogen.Therefore, the present invention overcomes this difficulty and producesa H₂/CO mixture with a molar ratio of at least 2 to 1, which is arequirement for the formation of methanol, which is achieved by using aspecific combination of steam and dry reforming of methane andsubstantially all of the hydrogen converted to methanol. As described inU.S. Pat. No. 7,705,059, this subsequent step can be performed, withoutlimitation, by direct catalytic conversion, or by a reaction, whichinvolves methyl formate as an intermediate.

The processes of the present invention allow for the substantiallycomplete utilization of carbon monoxide to form methanol or dimethylether. This represents an efficient and economical new way of methanolor dimethyl ether production, as well as an efficient new process forrecycling of carbon dioxide into methanol or dimethyl ether, thusrendering the carbon fuels renewable and environmentally carbon neutral.The process is not accompanied by any significant coke formation, aspresence of steam in the bi-reforming process retards coke formation andany carbon deposit still formed is in situ converted by reacting withCO₂ to form CO.

The processes of the invention to produce dimethyl ether also allow forrecycling of the water produced from the subsequent dehydration offormed methanol and do not require the use of external water.

As can be appreciated by one of skill in the art, the energy requiredfor the bi-reforming processes can come from any suitable energy source,including, but not limited to, excess energy fossil burning power plantsproduced in off peak use periods, any alternative energy sources, atomicenergy, etc. The bi-reforming process of methane or natural gas andcarbon dioxide to form dimethyl ether is an energy storage and fuelproducing process, but not one of energy production.

Any suitable source of natural gas or methane can be used, includingconventional natural gas sources, which can be produced, for instance,from “biogas,” a result of anaerobic bacteria's breaking down organicmaterial in the absence of oxygen. Biogas is produced in the digestivetracks of most mammals, organisms such as termites, and microorganismsduring digestion, as well as in wetlands, swamps and bogs, where largeamounts of rotting vegetation accumulate. Biogas is composed mainly ofmethane and carbon dioxide in varying proportions, and contains tracelevels of other elements such as hydrogen sulfide (H₂S), hydrogen,and/or carbon monoxide.

Any suitable source of carbon dioxide obtained from any available sourcecan be used, such as, carbon dioxide obtained from emissions of powerplants burning fossil fuels, fermentation processes, calcination oflimestone, other industrial sources, or even the atmosphere is utilizedvia its chemical recycling providing renewable carbon fuels intomitigating the environmentally harmful effect of excess CO₂. A carbondioxide source obtained from an exhaust stream from fossil fuel burningpower or industrial plant, or a source accompanying natural gas can beused. According to the process of the invention, carbon dioxide isrecycled instead of it being sequestered, which provides a way ofdisposal to the carbon dioxide produced by coal and other fossil fuelburning power plants and industries producing large amounts of carbondioxide.

The processes of the invention can also utilize carbon dioxide sourcefrom the atmosphere. Carbon dioxide content can be separated andabsorbed by using various processes as described in U.S. Pat. Nos.7,378,561 and 7,795,175 or can be recycled chemically as described inU.S. Pat. Nos. 7,605,293 and 7,608,743.

The processes of the invention can also utilize hydrogen derived from avariety of sources, including the electrolysis or cleavage of water. Onesource of hydrogen can be from the process of steam reforming of naturalgas, including, without limitation, in combination with the water gasshift reaction.

The processes of the invention can find multiple applications. Withoutbeing limiting, the combination of steam and dry reforming can be usedfor the recycling of CO₂ emissions from coal and other fossil fuelsburning power plants. It is also advantageous for use and recycling ofCO₂ from natural gas sources, which typically contain substantial CO₂concentrations. This is additionally practical, as CO₂ would, otherwise,have to be removed to allow further processing of the natural gas. Somenatural gas sources contain CO₂ concentration from 5 to 20%. Forexample, the natural gas at the Sleipner platform in Norway contains,for example, 9% CO₂. There, the CO₂ is currently already separated andsequestered beneath the North Sea in a deep saline aquifer. Other CO₂separation and sequestration processes are already being practiced inAlgeria and other locations, but sequestration is only a temporary,costly storage process with the release of large amounts of CO₂ whengeological events (such as earthquakes) occur.

Another application of the processes of the invention is to the use ofmethane hydrates. Methane hydrates are composed of methane trapped bywater in cage like structures called clathrates. Methane hydrates couldbe processed using a combination with a bi-reforming process where waterin the form of steam is added to react with methane. The transformationto syn-gas and further to methanol or dimethyl ether might render theexploitation of methane hydrates more economical.

EXAMPLES

The following examples illustrate the most preferred embodiments of theinvention without limiting it.

Example 1

A suitable molar mixture of CO₂, methane (or natural gas) and steam(water) to allow for a conversion of methane and CO₂ in excess of 90% isreformed in a single step in a flow reactor over a catalyst such as NiOat a temperature of about 800° C. to 850° C. to produce a gas mixturewith a molar ratio of approximately 2.05 moles of hydrogen to one moleof carbon monoxide. In this Example, the catalyst support is fusedalumina having a suitably large nanostructured surface. The NiO on fusedalumina support is quite stable for the reforming process.

Example 2

A mixture of methane, CO₂ and H₂O (3:1:2 mole ratio) is reacted over acatalyst composed of V₂O₅/NiO supported on nanostructural high surfacearea fused silica to give a hydrogen/carbon monoxide gas mixture closeto 2:1 suitable for the production of methanol.

Example 3

Hydrogen and carbon monoxide produced close to 2:1 ratio, as in Example1 and 2, are converted to produce methanol under catalytic reactionconditions using copper based catalysts.

Example 4

The methanol produced in Example 3 can be dehydrated to dimethyl etherusing a solid acid catalyst such as Nafion H between 100° C. to 200° C.

Example 5

The water formed during the dehydration of methanol to dimethyl ether isreacted with CH₄ and CO₂ in a 2:3:1 overall molar ratio in the one-stepbi-reforming process. With such water recycling dimethyl ether isproduced using methane and CO₂ in an overall ratio of 3:1.

Example 6

Methane and carbon dioxide in a mole ratio of 1:1 is dry reformed overNiO/V₂O₅ on fumed silica at 850° C. in a flow system to obtain a mixtureof hydrogen and carbon monoxide in an approximate 1:1 molar ratio.

The invention described and claimed herein is not to be limited in scopeby the specific embodiments herein disclosed, as these embodiments areintended as illustrative of several aspects of the invention. Anyequivalent embodiments are intended to be within the scope of thisinvention, as they will become apparent to those skilled in the art fromthe present description. Such embodiments are also intended to fallwithin the scope of the appended claims.

What is claimed is:
 1. A method of preparing methanol from carbondioxide and a methane source which comprises: combining a wet reformingof methane from the methane source with water and a dry reforming ofmethane from the methane source with carbon dioxide to form carbonmonoxide and hydrogen; combining the carbon monoxide and hydrogen fromthe wet and dry reforming without separation of components to produce amolar mixture of hydrogen and carbon monoxide in a specific ratio ofbetween 2:1 and 2.1:1, wherein the combining of the wet and dryreforming is combined in a single step and the methane, carbon dioxideand water are present in sufficient stoichiometric amounts to providethe recited molar mixture and ratio of hydrogen and carbon monoxide, andthe combining is conducted at a temperature of about 800 to 1100° C.;and converting molar mixture of hydrogen and carbon monoxide underconditions sufficient to exclusively form methanol, wherein the combinedwet and dry reforming is conducted in the presence of an added catalystthat includes single or mixed catalysts based on V, Ti, Ga, Mg, Cu, Mo,Bi, Fe, Mn, Co, Nb, Zr, La or Sn.
 2. The method of claim 1, wherein thecatalyst is supported on a high surface or nanostructured support. 3.The method of claim 2, wherein the support comprises silica, alumina, ametal oxide or a metal.
 4. The method of claim 1, wherein the methane,carbon dioxide and water are reacted in the single combined bi-reformingstep at a mole ratio of about 3:1:2 to provide the recited molar mixtureand ratio of hydrogen and carbon monoxide.
 5. The method of claim 1,wherein the molar mixture of hydrogen and carbon monoxide is present ata ratio of 2.05 to 1 and substantially all of the carbon monoxide andhydrogen reactants are exclusively converted to methanol.
 6. The methodof claim 1, wherein the catalyst is a catalyst combination of Ni andV₂O₅; Ni₂O₃ and V₂O₅; or Ni₂V₂O₇ and Ni₃V₂O₈.
 7. The method of claim 6,wherein the catalyst combination is supported on a high surface or nanostructured support.
 8. The method of claim 7, wherein the supportcomprises silica, alumina, a metal oxide or a metal.
 9. The method ofclaim 1, wherein the methane source is natural gas, coal bed methane ormethane hydrates.
 10. The method of claim 1, which further comprisesdehydrating the methanol thus produced to form dimethyl ether.